Vector network analysis relies on linear behavior from the device-under-test (DUT). However, some active devices must be stimulated in a unique manner to avoid nonlinear operation. To illustrate, a differential amplifier may exhibit distortion when driven with a single-ended signal. It is necessary to drive the inputs with real-time signals that present the proper amplitude and phase relationships. These drive signals must be presented at the input ports (+ and −) of the DUT, with the same amplitude and 180 degrees of phase difference, as a differential signal. For in-circuit applications, a balun (balanced to unbalanced transformer) is often used. It is positioned in close proximity of the device to avoid introducing any phase offset due to connections between the device and the balun. In application, it is difficult to control the interconnections to maintain the desired balance.